Development of current stand structure in dry fir-pine forests of eastern Washington

Abstract

Stand reconstruction, using live trees and deadwood, provided a four to five hundred year recruitment history for the current stand structure in dry fir-pine forests of northern Washington, USA. The lengthy structural record indicated these dry fir-pine forests were resilient to change, that all forest conditions were transient, and that forests were currently transitioning away from maximum tree density levels. Historically, frequent fires maintained low tree abundance, but fire cycles lengthened in the 1860s as Euro-settlement progressed. Average stand density had already increased to 194% (SD = 116) of 1860 levels by the start of effective fire suppression in 1915. From the 1930s to 1960s, average stand density peaked at 258% (SD = 98) of 1860 levels. By 2000, tree mortality in these overstocked stands had reduced average stand densities to 68% (SD = 19) of maximum levels. However, tree densities in 2000 still averaged 173% (SD = 64) above historical levels. Although there had been significant tree recruitment and a century without fire, the trees present in the historic stands provided 75% (SD = 13) of the basal area in 2000. Numerous understory (< 7.6 cm dbh) trees (252 trees/ha, SD = 97) were present in 1860 stands and these trees contributed more basal area than the historic overstory in 2000. In the absence of fire and without human intervention, the sampled stands have decreased in overall tree density from maximum levels while representation of shade-tolerant species, range and evenness of age-class structure, and abundance of old forest (live tree and deadwood) legacies have increased.